Bend insensitive fiber

An optical fiber comprising:

• a core comprising a maximum refractive index difference Δn1 in the optical fiber;
• an inner layer comprising a refractive index difference Δn2 that is smaller than the maximum refractive index of the core and decreases in a direction away from the core, the inner layer being positioned outside the core;
• and
• a trench layer comprising an inner-circumference refractive index difference Δn3 that is smaller than the refractive index difference of the inner layer and an outer-circumference refractive index difference Δn4 that is a minimum refractive index difference in the optical fiber, the trench layer being positioned outside the inner layer,
• wherein a refractive index difference of the trench layer gradually decreases from an inner surface to an outer surface of the trench layer, a ratio (Δn3/Δn4) of the inner-circumference refractive index difference Δn3 to the outer-circumference refractive index difference Δn4 is larger than 0.6 and smaller than 1, a ratio (a+b)/b of a radius ‘a’ of the core and a thickness ‘b’ of the inner layer is less than 2.8, and the optical fiber comprises a cutoff wavelength of 1260 nm or less, a bending loss α1 of 1.0 dB or less in bending by a radius of 10 mm or less at a wavelength of 1550 nm, and a ratio (α2/α1) of a bending loss α2 at 1625 nm to the bending loss α1 at 1550 nm being less than 2.7.